Input:

// test.ts
interface Xx {} 
export {Xx}

command:

> tsc --module system test.ts

output:

// test.js
System.register([], function(exports_1) {
    return {
        setters:[],
        execute: function() {
            exports_1("Xx", Xx); // Error: Can't find variable: Xx
        }
    }
});

In real use case I have to export interface because:
TS4073: Parameter 'callback' of public method from exported class has or is using private name 'ICallback'

This bug not occurs in 1.5.3, variable is not exported. So probably this is regression bug.